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Off-Grid Inverter Capacity & Large Loads

What used to be considered a sizable off-grid whole-house inverter 20 years ago (1,500 W continuous output power) is now considered a small inverter that would be suitable to serve cabin or RV loads. Today, inverter capacity of 4,000 to 8,000 W is common for residential-scale off-grid PV systems, and many of them can be stacked to provide 24,000 W or more. But just because higher-power inverters are available doesn’t mean that all high-power loads are appropriate for an off-grid home that relies solely on a PV system.

For example, electric heating loads should still be avoided. In this case, it is the runtime, or total energy use in kWh that we are concerned with. While a 6 kW inverter could power an electric water heater or some electric baseboard heaters, running those types of loads requires a larger PV array and battery bank. This strategy is generally not cost-effective. For example, a standard electric water heater would draw 10 to 20 kWh per day, requiring $15,000 to $20,000 of extra PV array and battery storage capacity. A $10,000 solar water heating system would probably be a better investment. Well pumps, which used to be the killer load for off-grid inverters, are no longer as hard on systems as they used to be. Much larger inverters are available that have better surge capacities than 12- and 24-volt ones (though not always). While this does mean that a 3/4 or 1 hp well pump is now operable without the help of a generator, you should still use the smallest well pump possible, since it will put less stress on the batteries and use fewer kWh per gallon of water.